I feel like things have changed as the main interface for code has (for some) become an agent running in the cli. I feel like we (certainly I) check my code editor way less frequently than before. Because of that (for me) easily reading/rendering Markdown files has become more of a pain than it used to be.
If you problem is that you don't have a text editor open...just open it to read the file? When I click on a Markdown file, it opens in my code editor with the preview pane already open. Then I close it when I'm done. How is that different from any other file on a computer?
If your problem is that you don't want to use a text editor, there are many markdown viewers out there, both dedicated (MarkLite), as part of a larger tool (Obsidian) or even in an office suite (LibreOffice Writer).
If your problem is that you don't want fo leave the terminal, there are many command line markdown "renderers", at least as far as that is even technically possible (glow is markdown-specific, bat is more of a general fancy text file viewer).
I fail to see how any of these problems are even partially solved by a web app and a CLI tool that launches it, let alone any better than the existing solutions.